erts: Fix bug in process_info(reductions)

returning incorrect result as
* current process might not be RUNNING in which case REDS_IN
  is actually used as def_arg_reg[5]
* FCALLS might not have been swapped out
* the SAVED_CALLS case was wrong and returned number of reds left

3 files changed, 52 insertions, 6 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index e909a0b4da..ce986fdd7a 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -3231,20 +3231,23 @@ erts_is_builtin(Eterm Mod, Eterm Name, int arity)
 
 
 /*
- * Return the current number of reductions for the given process.
+ * Return the current number of reductions consumed by the given process.
  * To get the total number of reductions, p->reds must be added.
  */
 
 Uint
-erts_current_reductions(Process *current, Process *p)
+erts_current_reductions(Process *c_p, Process *p)
 {
-    if (current != p) {
+    Sint reds_left;
+    if (c_p != p || !(erts_atomic32_read_nob(&c_p->state)
+                      & ERTS_PSFLG_RUNNING)) {
 	return 0;
-    } else if (current->fcalls < 0 && ERTS_PROC_GET_SAVED_CALLS_BUF(current)) {
-	return current->fcalls + CONTEXT_REDS;
+    } else if (c_p->fcalls < 0 && ERTS_PROC_GET_SAVED_CALLS_BUF(c_p)) {
+	reds_left = c_p->fcalls + CONTEXT_REDS;
     } else {
-	return REDS_IN(current) - current->fcalls;
+        reds_left = c_p->fcalls;
     }
+    return REDS_IN(c_p) - reds_left;
 }
 
 int
diff --git a/erts/emulator/beam/msg_instrs.tab b/erts/emulator/beam/msg_instrs.tab
index 9bf3aefaca..a6a0dec451 100644
--- a/erts/emulator/beam/msg_instrs.tab
+++ b/erts/emulator/beam/msg_instrs.tab
@@ -105,6 +105,7 @@ i_loop_rec(Dest) {
         $SET_CP_I_ABS(I);
         c_p->arity = 0;
         c_p->current = NULL;
+        c_p->fcalls = FCALLS;
         FCALLS -= erts_proc_sig_receive_helper(c_p, FCALLS, neg_o_reds,
                                                &msgp, &get_out);
         SWAPIN;
diff --git a/erts/emulator/test/process_SUITE.erl b/erts/emulator/test/process_SUITE.erl
index 57eb082d64..545dd95bef 100644
--- a/erts/emulator/test/process_SUITE.erl
+++ b/erts/emulator/test/process_SUITE.erl
@@ -44,6 +44,7 @@
          process_info_garbage_collection/1,
          process_info_smoke_all/1,
          process_info_status_handled_signal/1,
+         process_info_reductions/1,
 	 bump_reductions/1, low_prio/1, binary_owner/1, yield/1, yield2/1,
 	 otp_4725/1, bad_register/1, garbage_collect/1, otp_6237/1,
 	 process_info_messages/1, process_flag_badarg/1, process_flag_heap_size/1,
@@ -84,6 +85,7 @@ all() ->
      process_info_garbage_collection,
      process_info_smoke_all,
      process_info_status_handled_signal,
+     process_info_reductions,
      bump_reductions, low_prio, yield, yield2, otp_4725,
      bad_register, garbage_collect, process_info_messages,
      process_flag_badarg, process_flag_heap_size,
@@ -1084,6 +1086,46 @@ process_info_status_handled_signal(Config) when is_list(Config) ->
     false = erlang:is_process_alive(P),
     ok.
 
+%% OTP-15709
+%% Provoke a bug where process_info(reductions) returned wrong result
+%% because REDS_IN (def_arg_reg[5]) is read when the process in not running.
+process_info_reductions(Config) when is_list(Config) ->
+    pi_reductions_tester(spawn_link(fun() -> pi_reductions_spinnloop() end)),
+    pi_reductions_tester(spawn_link(fun() -> pi_reductions_recvloop() end)),
+    ok.
+
+pi_reductions_tester(Pid) ->
+    {_, DiffList} =
+        lists:foldl(fun(_, {Prev, Acc}) ->
+                        %% Add another item that force sending the request
+                        %% as a signal, like 'current_function'.
+                        PI = process_info(Pid, [reductions, current_function]),
+                        [{reductions,Reds}, {current_function,_}] = PI,
+                        Diff = Reds - Prev,
+                        {Diff, true} = {Diff, (Diff >= 0)},
+                        {Diff, true} = {Diff, (Diff =< 1000*1000)},
+                        {Reds, [Diff | Acc]}
+                    end,
+                    {0, []},
+                    lists:seq(1,10)),
+    unlink(Pid),
+    exit(Pid,kill),
+    io:format("Reduction diffs: ~p\n", [DiffList]),
+    ok.
+
+pi_reductions_spinnloop() ->
+    %% 6 args to make use of def_arg_reg[5] which is also used as REDS_IN
+    pi_reductions_spinnloop(1, atom, "hej", self(), make_ref(), 3.14).
+
+pi_reductions_spinnloop(A,B,C,D,E,F) ->
+    pi_reductions_spinnloop(B,C,D,E,F,A).
+
+pi_reductions_recvloop() ->
+    receive
+        "a free lunch" -> false
+    end.
+
+
 %% Tests erlang:bump_reductions/1.
 bump_reductions(Config) when is_list(Config) ->
     erlang:garbage_collect(),